serial network, all of the following must take place: Node A does the following: Enables its network driver. Sends the address of the node to transmit to. Sends the message. Disables the network driver and waits for a response. Node B does the following: Reads incoming data. Detects the node’s address. Reads the message associated with the address. Detects when the message has ended. Takes the requested action. Prepares a response. Enables the network driver. Sends the response. Disables the network driver. Node A then does the following: Reads the response. Takes any required action. At the same time, all of the other nodes must do the following: Read incoming data. Detect the transmitted address and ignore the message associated with the address. Protocols The nodes in a network must agree on a protocol for managing communications. Three types of network protocol are primary/secondary, token passing, and collision detecting. 268 Network Programming Primary/Secondary A primary/secondary protocol, also called the master/slave protocol, is often the least complex network protocol a network can implement. One computer is designated the primary node in charge of controlling all network traffic. The other computers are secondary nodes that respond to communications from the primary node. A network might have a PC as the primary node and embedded systems as secondary nodes. To give each node a chance to communicate, the primary node can poll, or send a message to, each of the secondary nodes in sequence. Each poll can request a response, which might contain an acknowledgment, requested data, an error message, or other information. A secondary node transmits only when the primary node has requested a response. Any message from one secondary node to another must pass through the primary node. The main limitation of the protocol is the delays that occur as each node waits to be polled. For a critical alarm system, waiting to be polled could be a problem, while a data-acquis prom-electric.ru